Field of the Invention
The present invention relates to an optical control device, and more particularly, to an optical control device having two or more signal electrodes as a modulation electrode.
Description of Related Art
Optical control devices have been used in which optical waveguides and modulation electrodes are formed in a substrate such as lithium niobate or the like having an electro-optical effect and optical waves propagating in the optical waveguides are modulated by the modulation electrodes. Recently, in order to meet requirements such as an increase in communication speed or an increase in communication data rate in the fields of optical communications or the like, plural Mach-Zehnder type optical waveguides (MZ-type optical waveguides) are driven using plural modulation signals in a differential quadrature phase shift keying (DQPSK) format.
In an optical control device having plural optical waveguides and plural signal electrodes, different modulation signals are often input to the signal electrodes and the different modulation signals are applied to the optical waveguides. Accordingly, when an electric field other than from a predetermined modulation signal acts on a specific optical waveguide, optical characteristics of the optical control device such as an extinction ratio of signal light degrade. This phenomenon is referred to as crosstalk.
In order to suppress the crosstalk between electrodes, it is effective to increase the gap between the optical waveguides or the gap between the electrodes, but this method causes an increase in device size, which is not desirable. As described in Japanese Laid-open Patent Publication No. 2009-53444, a method of forming a groove between optical waveguides or between signal electrodes has been proposed. However, since a process of forming the groove is particularly added to the optical control device manufacturing process, the manufacturing time and cost increase. In optical control devices using a substrate with a thickness of 30 μm or less, there is a problem in that a groove cannot be formed or that the substrate is easily destroyed even when the groove can be formed.
With an increase in frequency bandwidth of a modulation signal, a local potential difference corresponding to an operating state is generated between ground electrodes interposing a signal electrode therebetween. Accordingly, a phenomenon occurs in which a variation in electric field generated between a signal electrode and ground electrodes interposing the signal electrode therebetween differs on both sides of the signal electrode. When such electric fields are applied to the optical waveguides, an intended operation of modulation cannot be expected and modulation characteristics of the optical control device degrade considerably.